Both hydrostatic transmissions and geared transmissions are used in agricultural and construction equipment to transmit power from power sources, such as internal combustion engines to equipment for accomplishing a desired task. For example, transmissions are used to properly transmit power to the wheels of a vehicle, or to a vehicle implement. Two important considerations in selecting transmissions are their efficiency and range of input and output speed variability. In general, hydrostatic transmissions provide extremely high-speed variability between the input and output, but are less efficient than geared transmissions. Unlike construction equipment applications, the agricultural applications require prolonged operation at full power in the “tillage” or “working” speed range. Under these conditions, optimum efficiency is required. Further, “on road” haulage performance requirements similar to that of a truck is desired for operating the work vehicle on the highway. In certain instances, it is required that the work vehicle has the capability of maintaining a road speed of 50 km per hour.
In an agricultural equipment application, such as on a tractor, power to drive an implement is typically obtained from a power take-off (PTO) device that is coupled to the tractor's primary power source or engine. When the tractor is using the PTO, the engine speed must be held constant to maximize the efficient use of the implement. With a conventional transmission, such situation cannot be maintained if the tractor speed changes since that will affect the PTO's rpm and therefore affect the implement's efficiency.
Thus there is a need for a transmission for use with agricultural equipment that will provide a constant horsepower with the ability to change speed and torque in a seamless manner, in other words, “continuously variable”. There is also a need for a continuously variable hydro-mechanical transmission that provides a “working range” for tillage and heavy draw bar applications and a “roading range” for haulage and transport on the highway.